Facebook has >30m regular users via mobile phones

"There are more than 30 million active users currently accessing Facebook through their mobile devices.
People that use Facebook on their mobile devices are almost 50% more active on Facebook than non-mobile users.
There are more than 150 mobile operators in 50 countries working to deploy and promote Facebook mobile products"
Source: Facebook Press Statistics, retrieved on 9th April 2009.
This shows rapid growth from 21m mobile users in January 2009, and 15m in November 2008